Definition: Verongida in which the fibre skeleton when present is strongly anastomosing, frequently compressed into two dimensions, and radiating from a contracted base of attachment. Fibres contain cellular elements in concentric annuli; these occur mainly in bark elements, but can also be dispersed in pith as well. The fibres are of typical construction for the order with bark and pith elements represented, neither component is emphasised. Individual fibres can attain great thickness, particularly towards the base of the sponge. The skeleton makes up the major bulk of the sponge. A thick firm skin-like dermis is common to all species as is the inclusion of spherulous cells among the secretory cell content. The choanocyte chambers are large, and sac-shaped (Ianthella, Hexadella), to slightly ovate-elongate and occasionally branched (Anomoianthella). Colour in life ranges from sulphur yellow through deep orange, blue to deep purple or purple brown. All species show a characteristic oxidation reaction upon damage or death, and reach a final deep purple colouration. Soft tissue pigmentation is always uniform throughout the sponge.

Remarks: Three nominal genera, Ianthella, Anomoianthella, Hexadella.

Source: Bergquist & Cook, 2002g.

Genera represented in the area:HexadellaTopsent, 1896 (type species Hexadella racovitzaeTopsent, 1896): encrusting growth forms; no fibres or sponginspicules; ectosome is collagen reinforced to form a noticeable skin which, acting in some measure as an external skeleton, allowing the sponge to attain a thickness up to 5 mm.

Species included:Hexadellaracovitzai

Species not included:Hexadella dedritiferaTopsent, 1913: deep water North Atlantic